1. Field of the Invention
The present invention relates to an absolute position length measurement type encoder that includes a light-emitting element, a scale for forming bright and dark patterns with pseudorandom codes on projection light of the light-emitting element, and a light-receiving element, and in particular to an absolute position length measurement type encoder capable of carrying out highly accurate measurement in a stabilized state even if the scale is contaminated and/or the light-receiving amount is not even.
2. Description of the Related Art
Accurate position control and measurement are indispensable in measuring instruments and apparatuses. Therefore, an absolute position length measurement type encoder capable of executing absolute position measurement over some length has been used. In particular, where high accuracy is required, a photoelectric type encoder has been used.
Generally, an absolute position length measurement type photoelectric encoder has an absolute pattern (hereinafter called an ABS pattern) to roughly measure a moving distance and an incremental pattern (hereinafter called an INC pattern) to measure a moving distance at high resolution power by interpolating the interval of a roughly obtained moving distance on a scale (For example, Japanese Published Unexamined Patent Application No. 2003-254786 (hereinafter called Patent Document 1), Japanese Translation of International Application (Kohyo) No. 2005-515418 (hereinafter called Patent Document 2)). And, for example, the scale is attached to a moving stage side, and the light-emitting element and light-receiving element are attached to the base side that supports the stage.
And, since bright and dark patterns formed by ABS pattern and INC pattern, which are formed on the scale, change on the light-receiving element, which receives the bright and dark patterns, in line with movement of the stage, the moving distance can be measured at high accuracy by processing the change in a signal processing circuit. In detail, rough absolute positions are obtained by the ABS pattern, and a minute position between the absolute positions obtained by the ABS pattern is obtained by the INC pattern. That is, it is necessary that the absolute position itself obtained by the ABS pattern be stabilized with high accuracy. Also, a plurality of patterns whose cycles differ from each other are used as the ABS pattern, and the absolute positions are obtained by the phase relationship thereof. However, as the measurement distance is lengthened, such a problem arises in which the number of the above-described patterns is increased, and the scale and light-receiving element are large-sized. Accordingly, the absolute position length measuring type encoder itself can be downsized by using a single pattern based on a pseudorandom code as the ABS pattern.
However, where the pseudorandom pattern is used as the ABS pattern, in order to measure the absolute position, it is indispensable that the pseudorandom code is decoded from the signals produced from the ABS pattern. Therefore, it is necessary to carry out a binarization process. Patent Document 1 and Patent Document 2 disclose a method for the binarization process. For example, in Patent Document 1, signals produced by the ABS pattern are binarized by controlling the threshold values for the binarization process using bright and dark signals of the INC pattern. Further, in Patent Document 2, signals produced from the ABS pattern are binarized by comparing the outputs with each other in the light-receiving elements.
However, in these methods, the threshold values become unstable due to contamination of the ABS pattern and breakage of the ABS pattern as a practical problem, wherein a binarization error is likely to occur by which erroneous binarization is carried out, and resultantly there is a fear that measurement of an absolute position is not carried out in a stabilized state at high accuracy.